Deglacial mesophotic reef demise on the Great Barrier Reef (PDF ...

Palaeogeography, Palaeoclimatology, Palaeoecology 392 (2013) 473–494

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Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo

Deglacial mesophotic reef demise on the Great Barrier Reef E. Abbey a, J.M. Webster a,⁎, J.C. Braga b, G.E. Jacobsen c, G. Thorogood d, A.L. Thomas e,f, G. Camoin g, P.J. Reimer h, D.C. Potts i a

Geocoastal Research Group, School of Geosciences, University of Sydney, NSW 2006, Australia Departamento de Estratigrafia y Paleontologia, Universidad de Granada, Granada, Spain Institute for Environmental Research, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia d Institute of Materials Engineering, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia e University of Oxford, Department of Earth Sciences, Parks Road, Oxford OX1 3PR, UK f School of Geosciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK g Aix-Marseille Université, CNRS, IRD, CEREGE UM34, Europôle Méditerranéen de l'Arbois, B.P. 80, F-13545 Aix-en-Provence CEDEX 4, France h School of Geography, Archaeology and Palaeoecology (GAP), Queen's University Belfast, Belfast, BT7 1NN Northern Ireland, UK i Department of Earth and Evolutionary Biology, University of California, Santa Cruz, CA 95604, USA b c

a r t i c l e

i n f o

Article history: Received 5 June 2013 Received in revised form 27 September 2013 Accepted 30 September 2013 Available online 9 October 2013 Keywords: Mesophotic reef Submerged reef Coralgal assemblages Radiocarbon dating

a b s t r a c t Submerged reefs are important recorders of palaeo-environments and sea-level change, and provide a substrate for modern mesophotic (deep-water, light-dependent) coral communities. Mesophotic reefs are rarely, if ever, described from the fossil record and nothing is known of their long-term record on Great Barrier Reef (GBR). Sedimentological and palaeo-ecological analyses coupled with 67 14C AMS and U–Th radiometric dates from dredged coral, algae and bryozoan specimens, recovered from depths of 45 to 130 m, reveal two distinct generations of fossil mesophotic coral community development on the submerged shelf edge reefs of the GBR. They occurred from 13 to 10 ka and 8 ka to present. We identified eleven sedimentary facies representing both autochthonous (in situ) and allochthonous (detrital) genesis, and their palaeo-environmental settings have been interpreted based on their sedimentological characteristics, biological assemblages, and the distribution of similar modern biota within the dredges. Facies on the shelf edge represent deep sedimentary environments, primarily forereef slope and open platform settings in palaeo-water depths of 45–95 m. Two coral–algal assemblages and one non-coral encruster assemblage were identified: 1) Massive and tabular corals including Porites, Montipora and faviids associated with Lithophylloids and minor Mastophoroids, 2) platy and encrusting corals including Porites, Montipora and Pachyseris associated with melobesioids and Sporolithon, and 3) Melobesiods and Sporolithon with acervulinids (foraminifera) and bryozoans. Based on their modern occurrence on the GBR and Coral Sea and modern specimens collected in dredges, these are interpreted as representing palaeo-water depths of b60 m, b 80–100 m and N100 m respectively. The first mesophotic generation developed at modern depths of 85–130 m from 13 to 10.2 ka and exhibit a deepening succession of b 60 to N100 m palaeo-water depth through time. The second generation developed at depths of 45–70 m on the shelf edge from 7.8 ka to present and exhibit stable environmental conditions through time. The apparent hiatus that interrupted the mesophotic coral communities coincided with the timing of modern reef initiation on the GBR as well as a wide-spread flux of siliciclastic sediments from the shelf to the basin. For the first time we have observed the response of mesophotic reef communities to millennial scale environmental perturbations, within the context of global sea-level rise and environmental changes. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Mesophotic reefs are light-dependent coralgal communities found on deep forereef slopes (ca. 40–100 m) along continental margins and oceanic islands (Lesser et al., 2009; Kahng et al., 2010). Their distribution is becoming increasingly well-known as they are the topic of much

⁎ Corresponding author at: Geocoastal Research Group, School of Geosciences, The University of Sydney, NSW 2006, Australia. Tel.: +61 2 9036 6538; fax: +61 2 9351 0184. E-mail address: [email protected] (J.M. Webster). 0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.09.032

interest, thought to be refuges during past environmental disturbances (summarised in Bongaerts et al., 2010). These communities are commonly composed of depth generalists found in shallow-water reefs (Bongaerts et al., 2010) and are thought to be the source for shallowwater reef regeneration following disturbances. However, the genetic and ecological link between mesophotic communities and shallowwater reefs remains unclear (Van Oppen et al., 2011) and mesophotic reef presence in the fossil record is poorly documented compared with their shallow counterparts. Sea-levels dropped to a maximum level of about −125m during the Last Glacial Maximum (LGM) (Yokoyama et al., 2001; Peltier, 2002;

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Peltier and Fairbanks, 2006), and evidence for shallow-water reef colonisation on deep island flanks and continental shelf edges during the deglacial sea-level rise can be found in the South Pacific (Camoin et al., 2006; Cabioch et al., 2008; Flamand et al., 2008), Hawaii (Webster et al., 2004; Faichney et al., 2009), the Caribbean Sea (Macintyre et al., 1991; Toscano and Lundberg, 1999; Blanchon et al., 2002), the Indian Ocean (Wagle et al., 1994; Vora et al., 1996; Dullo et al., 1998; Rao et al., 2003; Camoin et al., 2004; Fürstenau et al., 2010) and Australia (Harris and Davies, 1989; Harris et al., 2004; Beaman et al., 2008; Woodroffe et al., 2010; Abbey et al., 2011a). Indications of rapid pulses in sea-level rise during the last deglaciation (meltwater pulses) have been identified in the submerged reef and coastal sequences in the Caribbean and the Indo-Pacific (Fairbanks, 1989; Hanebuth et al., 2000; Webster et al., 2004; Fairbanks et al., 2005; Camoin et al., 2012), with coral reefs responding to rapidly changing environmental conditions via a combination of community transitions, and/or complete demise and backstepping. The causes of shallow-water reef demise have increasingly been studied in both the modern (e.g., Eakin et al., 2010) and the fossil records (e.g., Montaggioni, 2005; Blanchon, 2011). However, the causes and occurrence of modern mesophotic reef demise are relatively unknown compared to their shallow-water counterparts (Smith et al., 2010). Mesophotic coral communities are similar in composition to shallow-water reefs (Bongaerts et al., 2010; Bridge et al., 2012), and as such are difficult to differentiate in fossil coral cores without the aid of multi-taxa reconstructions and precise radiometric dating. Mesophotic reefs also tend to have slow accretion rates (Grigg, 2006) and produce only a thin veneer of coral growth (e.g., Jarrett et al., 2005; Abbey et al., 2011b) and as such, there is limited potential to investigate fossil mesophotic reef death in vertically drilled sequences alone. Despite these limitations, fossil mesophotic reefs have the potential to provide valuable information about conditions during sea-level rise, as well as better constraining mesophotic tolerances. Due to its wide (50–150 km), mostly gently-sloping continental shelf reaching depths of N100 m (Hopley et al., 2007), the Great Barrier Reef (GBR) offers an excellent opportunity to study fossil mesophotic communities and their response to sea-level rise and palaeo-environmental changes. Modern mesophotic communities are found on submerged banks and Pleistocene reefs to depths of 75 m (Bridge et al., 2010, 2011a, 2011b; Harris et al., 2012), and provide a robust foundation for environmental reconstruction through direct comparisons of the fossil communities to the modern. Despite intensive study of the Holocene growth history of the modern GBR (see Hopley et al., 2007 for a comprehensive review), little is known of the submerged reefs found at the shelf edge. Submerged geomorphological features at depths of 50–130 m are interpreted to be the result of widespread reef growth during the deglaciation and previous periods (Harris and Davies, 1989; Beaman et al., 2008; Abbey et al., 2011a), but ecological and chronological information is sparse (Veeh and Veevers, 1970; Yokoyama et al., 2000; Davies et al., 2004). Prior to this study, only two corals have been recovered from these deep slopes, both in the southern GBR; a Galaxea clavus was recovered from 175 m depth and dated to 17.0 ka (Veeh and Veevers, 1970; Yokoyama et al., 2000), and an encrusting Acroporid was recovered from 90 to 110 m and dated to 9.1 ka (Davies et al., 2004). However, a recent program of offshore drilling on the shelf edge has targeted these submerged geomorphological features, and preliminary results confirm the underlying structure is composed of a combination of mainly shallow water coralgal-microbial framework and detrital facies that developed since the LGM (Webster et al., 2011). The deeper regions of the GBR shelf edge may provide new insights into the fossil mesophotic communities, their palaeo-environments and the timing and causes of their demise. Our study is based on samples and data collected on a 2007 cruise on the RV Southern Surveyor that investigated the geomorphology, fossil coral communities and modern benthic habitats preserved on the outer shelf of the GBR (Webster et al.,

2008). The specific objectives of our study are; (1) to describe the ecological and sedimentological characteristics of the fossil mesophotic communities and their palaeo-environmental significance; (2) constrain the timing of mesophotic reef demise and assess the cause of death during the last deglaciation; and (3) discuss the implications of these findings for understanding the environmental thresholds of these deepwater communities. 2. Location and methods The GBR extends from ca. 10° to 24°S along Australia's eastern continental shelf. Conditions are oligotrophic on the shelf edge where reefs grade from a nearly continuous barrier in the north to isolated platforms in the central region (Hopley et al., 2007). Shelf edge reefs are buffered from terrestrial influences due to their great distance from the shore (King et al., 2001; Brinkman et al., 2002). Four widely-spaced shelf edge sites on the GBR were selected for this study, including near Ribbon Reef 5, near Noggin Pass, near Viper Reef and near Hydrographers Passage (Figs. 1 and 2). Abbey et al. (2011a) conducted a detailed study of shelf geomorphology at these four sites, and identified drowned reef features including fringing reefs, patch reefs, an outer barrier reef and an inner barrier reef. Many of these features were dredged, including the following: 1. Continental slope: the slope seaward of the shelf break. 2. Shelf break: the inflection point demarking the continental slope from the continental shelf. 3. Terraces: flat, horizontal or sub-horizontal features bound on their landward and seaward margins by more steeply dipping sea bed. 4. Pinnacles: high relief, steep sided outcrops, generally circular to oval in shape and less than 100 m in diameter. 5. Barrier reefs: high relief outcrops with extensive linear continuity. They may be flat-topped or formed by closely-spaced or joined pinnacles.

2.1. Dredging Samples were recovered using a benthic sled, designed to recover the top layer of the substrate as it was towed over a distance of 50–250 m at each sampling site. Twenty-two dredges were recovered from between 46 m and 173 m (Table 1) with depth ranges estimated using a combination of shipboard GPS and 5 m pixel cell size bathymetric models (Bridge et al., 2010, 2011a; Abbey et al., 2011a). Depth errors were minimized (5–10 m) by dredging parallel to the isobath in most cases (Fig. 2). 2.2. Biota, facies and environmental characterisation Samples larger than ca. 50 mm in diameter were halved along their long axis and used for analyses and those smaller than 50 mm were not included. The cut surfaces of more than 900 selected samples were used to assess the facies, fossil assemblages and internal bioerosion. Modern biota were identified by the presence of live tissue and recent biota by preservation of fine-scale surface ornamentation but lacking tissue and/or a modern (b500years) radiometric age. The degree of bioerosion was estimated visually (Flügel, 2009) as a percentage of the cut surface area affected by voids created by boring organisms. Each sample was classified using Wright's (1992) revised version of Dunham's (1962) and Klovan and Embry's (1971) classifications. Samples were considered in situ on the basis of a freshly broken basal surface lacking any encrusting biota. Additional factors taken into account include the orientation of geopetals and the location of staining relative to the upper surface indicated by the biota (e.g. corallites). Those samples exhibiting rounding, no freshly broken basal surface or


A

B

C

475

D

Fig. 1. Four sites along the eastern margin of the Great Barrier Reef (GBR) were mapped using multibeam sonar (Abbey et al., 2011a) and dredged between depths of ca. 45–170 m. Regions include A) Ribbon Reef, B) Noggin Pass, C) Viper Reef and D) Hydrographers Passage. Details of boxed regions around dredges can be found in Fig. 2.

encrustations on a broken basal surface were interpreted to have been reworked and transported. The taxonomy and growth form of biota found within the samples were recorded, especially for corals, coralline algae, bryozoans and encrusting foraminifera. Exposed corallites and skeletal cross sections were used for identification of corals in conjunction with taxonomic guides (Veron et al., 1977; Veron and Pichon, 1979, 1982; Veron and Wallace, 1984; Veron, 1986, 2000). Seventy-four thin sections were used for the identification of coralline algae and encrusting foraminifera. In cut specimens, the maximum thickness of algal crusts was measured and the volumetric ratio of algae to foraminifera was estimated. Erect bryozoans were identified by zooecial chambers using a scanning electron microscope. The percent of the limestones' surface area encrusted was estimated visually; epibiont identifications were made; and their abundance was recorded. Models of vertical succession were constructed using a stratigraphic analysis of the fossil assemblage compositions, and placed within an absolute temporal context using radiometric dating.

2.3.1. AMS Radiocarbon measurement 2.3.1.1. OZ-samples. Sub-samples were extracted using a Dremel drill with a diamond wheel. Unlithified infill was removed in an ultrasonic bath using Milli-RO water and then organic matter was removed by treating with 10% H2O2 for 24 h. Etching with a dilute solution of HCl (0.125 N) removed 20–80% of each sub-sample to reduce secondary aragonite and high Mg-calcite. Coral samples were analysed for secondary calcite content before preparing samples for AMS analysis (Section 2.3.1.1.1). Samples 5–20 mg were then treated with H3PO4 (85%) at 60 °C overnight to release CO2, which was then converted to graphite by reduction with H2 over an iron catalyst at 600 °C (Hua et al., 2001). The graphite target was then analysed by AMS using a HVEE 2MV tandem accelerator at the Australian Nuclear Science and Technology Organisation (ANSTO). The measurements were normalised to an oxalic acid standard, corrected for background using IAEC C1 Carrera marble (Rozanski et al., 1992) and for fractionation (using δ13C measured separately on a Micromass IsoPrime IRMS with Elemetar Elemental Analyser) to give the conventional radiocarbon age (Fink et al., 2004).

2.3. Radiometric dating Radiocarbon dating by accelerator mass spectrometry (AMS) was the preferred method due to the small sample size necessary and the type of fossil to be dated. Four coral samples (including one replicate to total five measurements) were selected for U–Th dating to determine the local ΔR. In situ samples were preferentially selected, but the primary objective was to date a range of biota, including corals, coralline algae (geniculate and non-geniculate) and erect bryozoans. Pre-treatment for AMS was rigorous due to the degraded state of many of the samples, and the calcite content determined by XRD prior to preparing samples for AMS analysis.

2.3.1.1.1. XRD analysis. All pre-treated coral samples were powdered for X-ray diffraction (XRD) to quantify contamination and possible calcite recrystallization. The measurements were carried out using a PANalytical X'Pert Pro Diffractometer with Cu Kα radiation and collected over a 2θ range of 5° to 80°. About 50 mg of powdered coral was used for each test and aragonite standards with 0.1, 0.5, 2.0, 10.0 and 20.0% calcite were used for calibration. To test the efficiency of calcite removal, both pre-treated and untreated material from the same sample was analysed (sample OZL402). Calcite was reduced from 1.0% to 0.2% after 78% dissolution. All scleractinian corals comprise b 2% calcite after etching, with most of the samples b1% calcite.

476


Ribbon Reef Dr4 124 m

78 m 50 50

Dr2 Dr3

90

90

13

13

0

0

Noggin Pass Dr7 10

0

Dr6 154 m

Dr8

Dr5

97 m

60

10

100

0

133 m

Dr9

129 m 60 m

60

70

Dr11

13

70

0

Dr10

30

1

Viper Reef 70

90

90

Dr17

Dr20

Dr16

Dr18

98 m

15 mbsl

Dr14

121 m Dr15

80 m 70

224 m

Dr19

30

130

13

0

45

Hydrographers Passage

60 100

100

0

12

100 m

75

Dr26

Dr22 Dr24 61 m

90

120

Dr21

123 m

Dr25 98 m

105 120+

Fig. 2. Bathymetry (5× vertical exaggeration) overlain by dredging tracks (black lines). “Dr2” = Dredge 2. Dredge coordinates, distances and depth ranges can be found in Table 1. Distance of dredge tracks are labelled here in black text for scale, and contours are labelled in white. For a detailed description of the regional geomorphology see Abbey et al. (2011a).

2.3.1.2. UBA-samples. Coral samples were pre-treated following the method described in Burr et al. (2004). Calcite content was determined using XRD, to ensure samples contain b1% calcite. A small slab of coral was removed using a Dremel, and washed several times in Milli-Q

water using ultrasonication, then dried. Approximately 17 mg of the cleaned coral was transferred to a septa sealed vial and an appropriate amount of ~0.1 N HCl added to etch 50–60% of the sample, and allowed to react for 1–2 days. The remaining coral was washed several times

Table 1 Location, bathymetric range and morphologic setting of dredges. See also Figs. 1 and 2 for dredge locations. Site

Dredge

Location

Dredged distance (m)

Depth range (m)

Slope range (°)

Morphology of dredged area

Samples

Ribbon Reef

Dr2 Dr4 Dr3 Dr8 Dr9 Dr6 Dr5 Dr11 Dr7 Dr10 Dr18 Dr17 Dr16 Dr15 Dr20 Dr19 Dr14 Dr21 Dr22 Dr26 Dr25 Dr24

−15.3764/145.7966 to −15.3757/145.7967 −15.4893/145.8191 to −15.4904/145.8188 −15.3768/145.7988 to −15.3768/145.7983 −17.1052/146.5723 to −17.1052/146.572 −17.0919/146.5663 to −17.0908/146.5658 −17.1278/146.5861 to −17.1268/146.5859 −17.1262/146.5871 to −17.1268/146.5872 −17.0923/146.5723 to −17.093/146.573 −17.1034/146.5784 to −17.1027/146.578 −17.0238/146.5445 to −17.0238/146.5778 −18.8816/148.443 to −18.8822/148.4437 −18.8788/148.4464 to −18.8782/148.4458 −18.877/148.4492 to −18.8774/148.4503 −18.876/148.452 to −18.8765/148.4518 −18.8853/148.4859 to −18.8851/148.485 −18.8848/148.4865 to −18.8843/148.4854 −18.777/148.1983 to −18.7771/148.1963 −19.6948/150.2357 to −19.6797/150.2424 −19.7945/150.235 to −19.7975/150.2427 −19.787/150.4567 to −19.7866/150.4559 −19.7842/150.4589 to −19.7839/150.4583 −19.7297/150.3587 to −19.7299/150.3575

78 124 54 97 154 133 65 129 173 60 116 80 121 69 98 174 224 100 61 110 98 179

46–50 47–51 70–82 53–60 54–61 87–91 100–102 98–108 107–120 101–124 57 66–69 93–94 101–112 104–109 110–114 159–173 52–53 86–92 103–110 126–127 127–133

3.5–17.5 4.5–27 4.5–40.5 3–27.5 2–23.5 4.5–16 2–18 0.5–31 3.6–18.5 7.4–13.6 0.5–1.5 1.5–3.5 3–6.5 4.5–17 13.5–24 1.5–17 2.5–32.5 1–8 3–26 2–20 0.5–3 2.5–13.5

Outer barrier reef Outer barrier reef Shelf break and terrace Outer barrier reef Outer barrier reef Terrace rim Shelf break Upper slope Upper slope Upper slope Upper shelf Upper shelf Terrace Shelf break Shelf break Upper slope Submarine landslide Outer barrier reef Terrace rim Shelf break and terrace Upper slope ridge Upper slope

31 16 134 67 58 2 3 18 4 12 46 47 63 58 35 20 5 57 106 12 9 128

Noggin Pass

Viper Reef

Hydro Pass


with Milli-Q water then placed in a vial under vacuum until dry. The coral was reacted with 0.5 ml of H3PO4 (80%) at 90 °C until all the coral had dissolved. The carbon dioxide was transferred to the graphitisation reactor and graphitised in the presence of an iron catalyst at 560 °C for a maximum of 4 h according to the Bosch–Manning Hydrogen Reduction Method (Manning and Reid, 1977; Vogel et al., 1984). The 14C/12C and 13C/12C ratios were measured by accelerator mass spectrometry (AMS) on a 0.5 MV National Electostatics Corporation compact accelerator, at the 14CHRONO Centre, Queen's University Belfast, together with Icelandic spar samples for the background (blank) and TIRI turbidite secondary standards (Scott, 2006). The sample 14 12 C/ C ratio was background corrected and normalised to the HOXII standard (SRM 4990C; National Institute of Standards and Technology). For all radiocarbon samples the 14C age and 1 sigma error were calculated using the Libby half-life of 5568 yr following the conventions of Stuiver and Polach (1977). The ages were corrected for isotope fractionation using the AMS-measured δ13C, which accounts for both natural and machine fractionation. Conventional radiocarbon ages were calibrated using Calib rev.6.0.1 (Stuiver and Reimer, 1993) using the Marine 09.14c calibration curve (Hughen et al., 2004; Reimer et al., 2009), applying a locally derived regional marine correction (ΔR) of 8 ± 6 years (Druffel and Griffin, 1993; Druffel and Griffin, 1999). Paired measurements of U–Th and radiocarbon on four corals measured in this study suggest that ΔR may have been more variable. This extra variability will introduce some additional uncertainty to the calibrated ages for a few hundred years (ca. 200), but for the purposes of this study are not significant. Radiocarbon ages, calibrated ages and % calcite are reported in Table 8. 2.3.2. U–Th dating U–Th dating samples were subsampled with a diamond cutting wheel to avoid visible signs of alteration and bioerosion. Sample pretreatment consisted of ultrasonication in 18 MΩcm water to remove particulate contaminants. Sub-samples of 0.2–0.5 g were spiked with a mixed 229Th–236U tracer solution (Robinson et al., 2004) and dissolved with HNO3. Sample/spike equilibration was achieved by refluxing in aqua regia, drying down and dissolving twice in 15 N HNO3. Purification of U and Th was performed by anion exchange chromatography following a procedure adapted from Edwards et al. (1986). Mass spectrometric measurement of U and Th isotope ratios was by a Nu Instruments MCICP-MS, with minor isotopes 234U, 230Th and 229Th collected in an ion counter and all other beams measured in Faraday collectors. Instrumental biases were corrected using a standard-sample-standard bracketing approach with CRM-145 bracketing U samples and an in-house Th isotope standard for Th samples (Mason and Henderson, 2010). Isotope ratios are presented in Table 2 and U–Th ages are presented in Table 9. 3. Results 3.1. Taxonomy, growth form and distribution of biota The primary biological components within samples include corals, encrusting red coralline algae (CCA), erect and encrusting bryozoans, and encrusting foraminifera. Secondary fossil components include

477

calcareous green algae and benthic foraminifera. Living and recent biota were identified through the presence of soft tissue (corals reported by Bridge et al., 2011b), excellent preservation of surface ornamentation and/or a modern (b 500 years) radiometric age. Most samples are moderately encrusted (50% or more) with two or more epibionts within each dredge (see Fig. 6). Specimens of live encrusting coralline algae (CCA) were obtained from dredges ranging in depth from 45 to 130 m, and live corals were recovered from dredges ranging in depth from 45 to 100 m (Fig. 3). In total, four species, fourteen genera and seven Scleractinian families were identified and one unidentified family of Octocorallia. Eleven species, ten genera and four families of red algae were recognised and one green algae genus. Two genera of encrusting foraminifera and one genus of erect to platy bryozoans were identified (Table 3). Porites and Montipora are the most abundant corals and agariciids are common (Table 3). Coral morphology is dominated by the encrusting, platy, tabular and massive (domal) growth forms with rare branching corals (Fig. 4). Modern corals with encrusting or platy morphology have the widest depth range, and are found as deep as 100 m, but more commonly to a maximum of 80 m (Bridge et al., 2010, 2011b and data herein). Massive and tabular corals are more restricted and found to depths of 60 m. Fossil coral diversity is similar between sites with some minor variability in the distribution of the less-common corals: Galaxea were not recovered from Viper Reef, Cyphastrea were not recovered from Noggin Pass, Goniopora were not recovered from Hydrographers Passage, and Echinopora were only recovered at Noggin Pass (Table 4). Corals on the outer reef and upper shelf (ca. 45–60 m) had the highest taxonomic and morphologic diversity (Table 4), and diversity and abundance decrease with increasing depth. Modern algal crusts are dominated by the mastophoroid and lithophylloid sub-families (e.g., Hydrolithon, Neogoniolithon, Lithophyllum) and the melobesioid sub-family (e.g., Lithothamnion, Mesophyllum) with common Sporolithon and Peyssonnelia (Fig. 5). Similar to corals, CCA also exhibit depth zonation, with the mastophoroids/lithophylloids found to depths of about 55–60 m, and melobesioids found to depths of 95 m or deeper (Fig. 3). CCA are most commonly found at the outer barrier reef, and decrease in volume with increasing depth (Fig. 6). Acervulinids (e.g., Acervulinid sp. and Gypsina sp.) are not depth restricted but become the dominant non-coral encrusters with increasing depth. Modern encrusting (thin and lacey) bryozoans were not identified taxonomically, but they are most commonly found on the shelf break (Fig. 6). A thick (N2 cm) platy growth morphology was also present on the shelf edge, upper slope and shelf break and was identified as Celleporaria sp. Based on the taxonomic and morphologic observations of the modern biota, three distinct assemblages in relation to depth can be summarised (Table 5). 1. Massive/tabular corals: Similar to previously observed mesophotic corals (e.g., Reed, 1985; Bak et al., 2005; Bridge et al., 2010), these fossil corals exhibit a marked morphologic change in the transition from shallow to deep, whereby corals assume a flatter morphology at depths greater than 30 m. Corals between 45 and

Table 2 U-Series isotope data: Activity ratios are presented according to the decay constants of Cheng et al. (2000, their Table 3). Dating ID

Dredge (mbsl)

[238U] (ppm)

2σ

[232Th] (ppb)

2σ

(230Th/238U)

2σ

(234U/238U)

2σ

(232Th/238U)

2σ

D4RR2ia D4RR2ib D22HP2ia D22HP4ia D22HP13iaa D22HP15ia

D4 (47–51) D4 (47–51) D22 (86–92) D22 (86–92) D22 (86–92) D22 (86–92)

3.3782 3.5775 3.8946 2.5475 0.46030 2.4559

0.0012 0.0005 0.0005 0.0004 0.00011 0.0003

20.92 10.55 12.37 0.0670 2.504 2.403

0.14 0.02 0.08 0.0004 0.016 0.016

0.004700 0.001431 0.1055 0.1204 0.1846 0.1187

0.000018 0.000019 0.0004 0.0004 0.0007 0.0004

1.1466 1.1454 1.1419 1.1417 1.1330 1.1402

0.0009 0.0009 0.0009 0.0009 0.0009 0.0009

2.03E−03 9.65E−04 1.04E−03 8.62E−06 1.78E−03 3.20E−04

1.3E−05 2.2E−06 6.8E−06 5.7E−08 1.2E−05 2.1E−06

a

Specimen was a bryozoan, all else were coral.


algae

depth (m)

Peyssonnelia sp.

Sporolithon sp. S. molle

L. insipidum

Lithophyllum cuneatum

Pavona sp.

Montipora sp. Porites sp. Agaricid (undet.) Leptoseris sp.

Pachyseris sp.

platy/ encrusting

Galaxea astreata G. paucisepta Favid (undet.)

massive/ tabular Acropora sp. Montipora sp. Porites sp.

Seriatopora sp.

Pocillopora sp.

branching

Melobesioid

Lithophylloid

Hydrolithon sp. H. breviclavium H. reinboldii Neogoniolithon fosliei Lithoporella sp.

Mastophoroid

Echinopora sp.

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140

Melobesiod (indet.) Lithothamnion muelleri (?) L. prolifer Mesophyllum sp. M. funafutiense

corals

L. acrocamptum L. gr. pustulatum

478

Fig. 3. Modern coral and coralline algae distribution determined from dredges. Corals and algae were either living when collected or identified as modern through AMS and U–Th dating. Depth ranges are only determined from those corals and coralline algae which were collected in situ. Grey bands indicate depth ranges which were dredged. Major coral and algae groups are labelled.

60 m are primarily massive or tabular (flat and thick), especially Porites, Montipora and faviids. The CCA within this depth range are dominated by lithophylloids and secondary or minor mastophoroids. 2. Platy/encrusting corals: At depths greater than 60 m corals are much thinner (b2 cm), with platy and encrusting morphologies, especially

Porites, Montipora and agariciids. CCA are dominated by melobesioids and Sporolithon. 3. Non-coral encrusters: At depths greater than 100 m, biota include a range of octocorals (Bridge et al., 2011b) and algal-foraminiferal communities. CCA include Peyssonnelia and Sporolithon to the exclusion of all lithophylloids and mastophoroids.

Coral morphology

Biota Ribbon Reef

Noggin Pass

Viper Reef

Hydro. Pass.

Ribbon Reef

Noggin Pass

Viper Reef

Hydro. Pass.

Outer barrier reef and upper shelf (46-69 m) Terrace rim (86-92)

no corals

no identifiable biota

Terrace top (93-94 m)

Shelf break (70-112)

no corals

no corals

Upper slope (98-133 m) Deep ridge (127-130 m) Submarine landslide (159-173 m)

no corals

Coral

CCA

Acervulinid

Bryozoan

Branching Massive Tabular

Platy Encrusting

Fig. 4. Distribution of internal biota and coral morphology by geomorphologic feature. External non-coral encrusting biota are addressed in Fig. 6.


A

B

C

D

479

500 µm

Fig. 5. Representative species of coralline red algae. (CCA) A) D4-1 Lithophyllum insipidum (modern); B) D4-3 Lithothamnion prolifer with conceptacles (modern); C) D8-56 Lithoporella sp. (modern?); D) D22-4 Mesophyllum funafutiense (fossil).

3.2. Facies and environmental interpretation Using sedimentary and textural observations in conjunction with fossil assemblages, six in situ (autochthonous) facies and five detrital (allochthonous) sedimentary facies have been identified (see Table 6 for detailed descriptions). Facies can be grouped into three primary categories based on their genesis which include: 1) boundstones (including six sub-facies, Table 6; Fig. 7D), indurated boundstones (Fig. 7A) and isolated colonies; 2) macroids and rhodoliths (Fig. 7C); and 3) allochthonous grainstones, floatstones, rudstones, shellstones and calcimudstones (Fig. 7B). The environmental interpretations of the facies are based on their mode of genesis, characteristic fossil components and the modern distribution of similar facies across the shelf (see Table 7 for details). The composition and genesis of facies is a useful indicator of palaeoenvironment and water depth when modern analogues can be identified. The use of analogous coral and coralline algae distribution for palaeo-

Outer barrier reef

Encrusting coralline algae Bryozoa

water depth reconstructions is a well-established methodology (Lighty et al., 1982; Adey, 1986; Pirazzoli and Montaggioni, 1988; Cabioch et al., 1999). 3.2.1. Boundstones and isolated colonies Boundstones and isolated colonies have the most diverse composition of the facies and have a widespread distribution across the shelf edge. They are most commonly found on the upper slope, shelf break or the outer barrier reef (Fig. 8). The interpretation of the coral/coralgal boundstone and isolated colony facies is based on the distribution of the modern coral and algae analogues. Coral and coralgal boundstones and isolated colonies generally comprise one of the coralgal assemblages identified in the modern biota (Table 5). Coralgal boundstones are considered photophilic, as each component is dependent upon irradiance for metabolism. As such, all coralgal boundstones have an interpreted depth range of b60 m, and b 80–100 m which is consistent with the observed photic

Acervulinid foraminifera

Serpulid worms

Abundance Scale

50-100% of surface covered

Upper shelf Terrace top Terrace rim Shelf break

25% of surface covered

Upper slope Deep ridge Submarine landslide

0% of surface covered

Fig. 6. Non-coral encruster distribution by geomorphological feature and percent of sample surface covered. The triangle at the right indicates the scale for spindles at the left, ranging from 0% to N50% of the sample's surface covered by each individual biota group.

480


order of overgrowth and vertical succession discussed in more detail below. Modern algal-foraminiferal boundstones were not found to be depth-restricted (within the 130 m depth sampling range) and therefore provide little depth constraint for the fossil assemblage. However, acervulinids are poor competitors for space and where they dominate, it is usually due to the reduction of CCA in low-light, cryptic environments (Perrin, 1992; Flamand et al., 2008). Modern platy bryozoans were not recovered in dredges and therefore their modern distribution on the GBR is unconstrained. However, Celleporaria sp. is found along the southern margin of Western Australia on silty substrates in low-energy, mesotrophic environments (Hageman et al., 2003). Their development is especially supported during low sea-levels when upwelling and lower surface temperatures favour a well-mixed water column.

Table 3 Taxonomy of identified biota, modern (superscript ‘M’) and fossil (superscript ‘F’). Coral

Coralline and other calcareous algae

Family ACROPORIDAE Undet.F Acropora sp.FM Montipora sp.FM

Family CORALLINACEAE Sub-family MASTOPHOROIDEAE Hydrolithon sp.M Hydrolithon breviclaviumM Hydrolithon reinboldii (?)FM(?) Hydrolithon rupestre?F Lithoporella sp.FM Neogoniolithon foslieiM Spongites sp. (?)

Family AGARICIIDAE Undet.FM Leptoseris sp.M Pachyseris speciosaFM Pavona sp.M

Sub-family LITHOPHYLLOIDEAE Lithophyllum gr. pustulatumFM Lithophyllum acrocamptumM Lithophyllum insipidumM Lithophyllum cuneatum Lithophyllum sp.FM Paulsilvella sp.F

Family ALCYONIDAE Lobophytum sp.M Family FAVIIDAE Undet.FM Cyphastrea sp.F Cyphastrea chalcidiumF Echinopora sp.FM Fungia sp.M

3.2.2. Indurated boundstones The indurated boundstones generally have little identifiable biota and none that is modern, but are characterised by dense, lithified pelagic or hemipelagic sediments within the skeletal interstices, borings and between algal and foraminiferal crusts (Fig. 7A). Indurated boundstones are found on the shelf break at depths of 100 m or more (Fig. 8). These sediments, combined with the algal and foraminiferal crusts, indicate deep conditions with limited terrigenous input (Flügel, 2009) (assumed by the pelagic origin of sediments) and occur on deep, open platform settings.

Family HAPALIDIACEAE Sub-family MELOBESIOIDEAE Undet.FM Lithothamnion sp.FM Lithothamnion muelleri (?)M Lithothamnion proliferM Mesophyllum sp.FM Mesophyllum funafutienseFM

Family OCULINIDAE Galaxea sp.M Galaxea astreataFM Galaxea pauciseptaFM Family POCILLOPORIDAE Pocillopora sp.M Seriatopora sp.M

Family SPOROLITHACEAE Sporolithon sp.M Sporolithon molleM

Family PORITIDAE Undet.F Goniopora sp.F Porites sp.FM Bryozoan Family CELLEPORARIIDAE Celleporaria sp.

Family PEYSSONNALIACEAE Peyssonnelia sp.FM

3.2.3. Macroids and rhodoliths These coated structures form through successive episodes of encrustation and repeated repositioning or turning, as they are unattached to the seabed. This movement can be a result of near-bottom currents, bioturbation or a combination of the two (Bosence, 1983; Harris et al., 1996). Macroids and rhodoliths are not commonly a dominant facies across the shelf edge within this study, but are the most abundant facies at Viper Reef on the upper shelf from 55 to 70 m (Fig. 8) where substrates are probably more mobile. Rhodoliths have also been observed on the Queensland shelf to depths of ca. 120 m, but are more commonly found down to about 90–100 m (Harris et al., 1996; Marshall et al., 1998; Lund et al., 2000). The algal components of the macroids (Lithothamnion, Mesophyllum funafutiense and Lithoporella, Table 6) identified here are similar to those found within rhodoliths.

Foraminifera Family ACERVULINIDAE Acervulina sp. Gypsina sp.

FM Taxonomic identification based on fossil or modern specimens. F = fossil only, M = modern only, FM = both fossil and modern specimens identified.

zone (Hopley et al., 2007) and their modern distribution (Table 5). When boundstones comprise a coralgal assemblage and components, such as foraminifera and bryozoans, the interpretation is based on the

Table 4 Coral taxon distribution by dredge. The number of asterisks (*) indicates relative abundance. Coral family Site

Dredge Undet Acroporidae

Agariciidae

Undet Acropora Montipora Undet. Pachyseris speciosa Ribbon Reef

Dr2 Dr3 Dr4 Noggin Dr7 Pass Dr8 Dr9 Dr11 Viper Reef Dr15 Dr16 Dr17 Dr18 Dr19 Hydro Dr21 Pass Dr22 Dr24

** **** ** **** ** *** * *** *** ** * **** ***

*** * *

***

*

*

*

* * ***

Favidae

** * ***

* ** *

* **

** * **

* ****

** *

* *

*

****

** **** * * *

*** *** ** ** *

** * ** * **

** * *

Oculinidae

Undet Cyphastrea chalcidium

* *** ** ** *** *

Poritidae

Echinopora Galaxea

Undet Porites Goniopora

paucisepta astreata

*

* ***

*

***

* *

*

* *

*** *

** *

* * *

**** *** **** ** ** ** * ** ** ** *** *** **** *** ***

*

*

*


481

Table 5 Coralgal and non-coral encruster assemblage characteristics and modern distribution. Assemblage

Characteristics

Modern depths

Massive/tabular corals

Dominated by massive and tabular (N2 cm thick) corals, especially Porites, Montipora and Acropora with associated encrusting and platy growth. Red coralline algae (CCA) are 1–10 mm thick and diverse. Every coralline observed is present within this depth range, and common genera include Peyssonnelia, Lithothamnion (L. muelleri), Lithophyllum (L. insipidum) and minor Hydrolithon (e.g., H. reinboldii, H. breviclavum). Dominated by thin (b2 cm) encrusting and platy corals, especially Porites, Montipora and agaricids associated with thin crusts (≤1 mm) of CCA, especially Sporolithon and Melobesioids (Mesophyllum funafutiense).

Depths of 60 m or less (Bridge et al., 2010, 2011a), but when massive Favids are dominant this assemblage can extend to 70 m. These algae are found across a wide variety of water depths from b 40–117 m, but Lithophyllum becomes rare below 60 m (Marshall et al., 1998; Lund et al., 2000).

Platy/encrusting coral

Non-coral encruster

Thin and encrusting CCA interlaryered primarily with acervulinid foraminifera and minor bryozoan.

3.3. Chronology Radiocarbon and U–Th dating from fifty-four specimens, including corals, CCA and erect bryozoans, reveal the fossil reef components distributed across the shelf edge range in age from about 16 ka to modern, with ages clustering mainly between 9 and 13 ka, 5–8 ka and 0–2 ka (Tables 8 and 9). The ages within a single dredge can vary by up to 5.5 ky, but are usually within 2–3 ky (Fig. 10). Several corals underwent dual radiocarbon-uranium-series dating or replicate radiocarbon dating to ensure reproducibility, and validate the reservoir correction. All replicate radiocarbon ages are consistent within a 2σ error. One dual radiocarbon–uranium series coral date

Depths of 100 m, but usually less than 80 m. Similar platy corals have been observed across many regions of the shelf to depths of 80 m or more (Harris et al., 1996; Hopley et al., 2007), but coral cover is extremely low at these depths (Bridge et al., 2010; Bridge et al., 2011b). Similar algal crusts are common to depths of at least 117 m. (Harris et al., 1996). Beyond 60 m, Sporolithon and Peyssonnelia become dominant (Marshall et al., 1998; Lund et al., 2000) Depths greater than 100 m within boundstone facies, or 55–70 within the macroid facies (see Table 6 for facies descriptions).

varies by more than 2σ, though the difference is not significant for the purposes of this study. Most chronology was performed on the boundstone facies, with some dates from detrital facies as well. Components that were radiometrically dated (directly dated) and those components observed within the same boundstone (indirectly dated) include corals, bryozoans (both erect and encrusting), acervulinids and CCA. CCA are the oldest fossil components directly or indirectly dated, and are also the most persistent, spanning the longest time period. They occurred within indurated boundstones from 16 to 14 ka on the submarine landslide at 159–172 m and then within coral–algal boundstones (indirectly dated) as well as within foraminiferal–algal

Table 6 Facies descriptions. Facies Autochthonous facies Boundstones (BD)

Isolated colonies (IC)

Rhodoliths (RH)

Macroids (MC)

Indurated boundstones (IBD)

Crystalline (CN)

Allochthonous facies Grainstones (GR) Floatstones (FL)

Rudstones (RD) Shellstones (SH) Calcimudstones (CM)

Description The boundstone facies (Wright, 1992) is composed of multiple layers (2+) of encrusting and binding organisms (e.g., Fig. 7D). Growth hiatuses are indicated by a layer of mud, a new crust (ECA, Acervulinid or bryozoan) or a transition to a new coral taxon. This facies comprises seven sub-facies.

Sub-facies Coral (BDc) Algal (BDa) Coralgal (BDca) foraminiferal (BDf) Foraminiferal–algal (BDfa) Foraminiferal–coral (BDfc) Bryozoan (BDb)

In the case of corals and bryozoans, when a growth hiatus is not identified, the specimen is characterised as an isolated colony. Isolated colonies of corals and bryozoans can reach dimensions of up to 51 cm and 25 cm across their longest axis, respectively. Rhodoliths are a type of coated grain (oncoid) that comprises an inner bioclastic nucleus, often a coral or bryozoan fragment, and concentric outer crusts. Rhodoliths are encrusted exclusively by algae. Specimens have an average long-axis length of 6.1 ± 2.4 cm. Similar in genesis to a rhodolith, macroids are encrusted predominantly by foraminifera (Acervulinids) with varying additional components, often algal and bryozoan (Fig. 7C). Specimens have an average long-axis length of 8.2 ± 2.1 mm Indurated boundstones are boundstones that have had most, if not all skeletal pore space and bore traces infilled with peloidal and hemipelagic sediments and lithified. Evidence of the original binding biota may remain in the form of light-coloured intercalated layers (e.g. Fig. 7A) and occasional skeletal preservation, but most often the biota is unidentifiable. The rock itself, as well as any biota, is stained to a dark rusty orange to dark brownish-black colour. Density is relatively very high compared to all other facies. The crystalline facies has no identifiable texture and is highly bored and infilled with unlithified sediments. The original facies cannot be identified and for simplicity only, it is grouped within the autochthonous facies.

Grainstones are mud-free with grains larger than 1 mm, most often comprising skeletal grains and foraminiferal tests. Floatstones are moderately to poorly sorted facies comprising N 10% of grains larger than 2 mm and are matrix-supported. They are composed primarily of the disarticulated plates of the green calcareous algae, Halimeda, and associated foraminiferal tests and skeletal grains (Fig. 7B). Bivalves are commonly a primary or secondary component. Rudstones are similar in composition to floatstones, but are differentiated by their grain-supported texture. Shellstones are grain-supported with more than 75% of grains comprising shells (usually bivalve) and often incorporating large coral plates. Grains are often large (cm-size) bivalves cemented together. Calcimudstones are rare and contain little if any identifiable biota and are dominated by sandy carbonate mud.

482


A

B

D

C

e

a

e

c c

e

e c

Fig. 7. Representative examples of an A) indurated boundstone facies comprising laminated layers of CCA (white layers) and hemipelagic mud (dark layers); B) floatstone facies comprising disarticulated Halimeda plates in a fine-grained matrix; C) macroid facies comprising a nucleus with successive overgrowths of thin CCA (e) and thick acervulinids (a); D) coralgal boundstone facies comprising successive overgrowths of corals (c) and CCA (e).

boundstones (directly dated) from 13 to 10.1 ka at 95–130 m. From 9.5 to 9.1 ka algae again occurred within indurated boundstones at 100 m. The longest gap in direct or indirect dating occurs from 9.1 to 7.8 ka. From 7.8 ka to present, CCA have grown within algal and coral–algal boundstones at N 60–80 and 95 m, and within indurated boundstones at 105 m. Corals within coral and coral–algal boundstones occurred on the shelf from 13 to 10.1 ka at 95–130 m. Corals had both the earliest and latest occurrence at Hydrographers Passage, spanning the entire range from 13 to 10.1 ka. At Noggin Pass, coral presence overlapped with that at Hydrographers Passage 11.3–10.2, and at Viper Reef from 12.3 to 11.1ka. No corals were dated (directly or indirectly through overgrowths) from 10.1 to 7.8 ka at any depth across the shelf, but were again within coral–algal boundstones at 7.8 ka at 60 m. Following this apparent hiatus, some corals developed at greater depths of 95–100 m from 7.8 to

present, but most were primarily within the depth range of the modern mesophotic communities at b80 m. Detrital facies and large platy bryozoans were rarely dated. The earliest occurrence of bryozoan boundstones was 13 ka at 95 m and they were present through to 9.5 ka to depths of 130 m. These fossil platy bryozoans were found primarily at Hydrographers Passage and no modern equivalents were observed following the hiatus. Detrital floatstones and rudstones comprising Halimeda plates were dated to 11.8–11.0 ka at depths of 100–130 m, and again at 7.2 ka at 95 m. 3.4. Vertical biologic succession Small-scale, local biological succession can be observed in the form of encrusting overgrowths within a single sample, usually a boundstone. The patterns of vertical succession are a useful record of environmental


and ecological change, especially in instances where radiometric dating is unavailable. Three distinct overgrowth patterns are common across the shelf and can be characterised by their photophilic or cryptic biota and transitions from one into another (Fig. 9). Using these successions, the ecological trajectory can be used to better understand changing environments (e.g., deepening or changing water clarity). 1. The first pattern is characterised by a stable photophilic succession of corals from the massive/tabular coralgal assemblage (i.e. corals and CCA interlayered, Fig. 9A). Corals are overgrown by other corals or 5+ mm thick CCA. The coralgal assemblage (Table 5) present in this succession is restricted to 60m based on the modern distribution (Fig. 3). When this assemblage persists through time as a succession, it is indicative of relatively stable mesophotic environments within 60 m water depth. 2. The second pattern is characterised by the succession of photophilic corals of the platy and encrusting coralgal assemblage (Table 5) into more cryptic biota (i.e. corals and CCA, then acervulinids or bryozoan, Fig. 9B). This succession from photophilic to cryptic suggests progressively deepening environments, but the mesophotic coralgal assemblage indicates an initial depth that is still within the photic zone. Palaeo-water depths based on the assemblage interpretation are most likely greater than 60 m in order to exclude the massive corals, and probably 80–100 m. 3. The third pattern consists of the stable cryptic succession of the noncoral encruster assemblage (i.e. CCA, acervulinids and bryozoans, Fig. 9C). The lack of corals indicates that sea-level rise has submerged surfaces to depths greater than 100 m. This cryptic succession, comprising low-light tolerant CCA and fully heterotrophic biota (acervulinids and bryozoans, Table 5), has also been found on the deep forereef slopes of New Caledonia in water depths of 110–160 m (Flamand et al., 2008). These successions are distributed evenly across each of the four sites, but vary with depth. In the very deepest dredge at 159–172 m, CCA are intercalated with peloidal and hemipelagic infill in a stable cryptic succession. On the upper slope from about 100–130m, all three patterns of vertical succession are apparent, and on the shelf break from 100 to 110 m, CCA are overlain by dark, indurated pelagic sediments in a stable cryptic succession. On the terrace top and rim from 85 to 95 m, successions of stable photophilic corals and CCA as well as photophilic to cryptic corals encrusted by thick bryozoans are present. On the upper shelf and outer barrier reef from 45 to 60 m, corals and CCA alternate in thick layers for many generations in a stable photophilic succession. 4. Discussion 4.1. Mesophotic reef growth and succession across the shelf edge The sedimentology, palaeoecology and radiometric data from the continental margin of the GBR indicate a more widespread, diverse and temporally dynamic fossil mesophotic reef system than previously recognised. These communities persisted at depths which up until now have been poorly constrained in fossil mesophotic systems, and as such they provide a unique perspective on marginal habitats during periods of lower sea-level and rise. Based on a synthesis of geomorphologic, sedimentologic and palaeoecologic data, we have reconstructed the range of depositional environments across the shelf edge. Combined with a comprehensive chronologic framework, we can now place the development of these fossil mesophotic communities within the context of sea-level rise and environmental perturbations, to better understand their environmental thresholds. 4.1.1. Response and succession at 100–130 m Samples at a modern depth range of 100–130 m exhibit all three observed patterns of vertical succession (Section 3.4), including

483

mesophotic coral community development in stable conditions, a transition into a cryptic environment, and the sustained development of cryptic biota after deep submergence (Fig. 9). Radiometric dating (Table 8), palaeo-environmental interpretations (Table 5), and reconstructed sea-level (see Yokoyama et al., 2006 for a discussion on possible biases) are consistent with mesophotic and mesophotic-cryptic community development in palaeo-water depths of b60–70 m from 13 to 10 ka (Fig. 11). From 10 ka to the present, exclusively cryptic communities had replaced the coral assemblages. The palaeo-environmental interpretation of the cryptic biota (Table 5), similar observations of cryptic biota on New Caledonia (Flamand et al., 2008) and known sea-level, are consistent with this community developing at depths of 80–130 m during the last 10 ka. 4.1.2. Response and succession at 85–95 m At modern depths of 85–95m only two patterns of succession within limestones were observed. A stable mesophotic community developed in palaeo-water depths of b60 m from 12 to 10 ka. Based on the coralgal assemblages, palaeo-water depths exceeded 60m by 10ka. No corals are recorded again at this depth until 5.5 ka, where communities shifted into the transitional mesophotic-cryptic community dominated by platy and encrusting corals (Fig. 11). 4.1.3. Response and succession at 45–60 m At modern depths of 45–60m, steady photophilic succession (Fig. 9) is coupled with the massive coralgal assemblage (Table 5). The earliest age of mesophotic coral development for this surface is 7.8 ka, which is consistent with the palaeo-water depth interpretation of the assemblage and known sea-level during the last ca. 8 ka (Fig. 11, Table 5). This massive coralgal assemblage continues to be widespread across the shelf within the modern mesophotic communities (Bridge et al., 2010, 2011a, 2011b). 4.2. Mesophotic community generations Based on observations of community composition, development, vertical succession patterns and chronology, we have identified two distinct generations of mesophotic coral growth across the shelf. The first generation began by 13 ka and ended at 11–10.2 ka, characterised by the shift from mesophotic coralgal assemblages into transitional or wholly cryptic non-coral assemblages at depths of 85–130 m (Fig. 12) at Hydrographers Passage, Noggin Pass and Viper Reef. A drill core coral record from the GBR also shows indications of a hiatus ca. 10 ka. IODP Expedition 325 penetrated the dredged surfaces at depths ranging from ca. 50 to 130 m. Sixty-seven corals from eighteen cores drilled at three of the four study sites have been dated using U–Th (Webster et al., 2011), and corals within 1 m of the seafloor range in age from ca. 13.5–9.7 ka (ages from unconsolidated facies were excluded). Across the shelf and indicated by both dredges and drill cores, a 2 ky hiatus in mesophotic coral growth occurred between 10.2 and 7.8 ka when communities were dominated by the deep non-coral, encrusting algae-foraminiferal assemblage. The hiatus ended with the development of a second mesophotic coral generation at 7.8 ka, where similar coral communities to those of the first mesophotic generation reformed upslope at depths of 45–95 m. 4.3. Causes for mesophotic coral hiatus Shallow-water reef generations, (sensu Montaggioni, 2005, also observed in Marquesas Islands, French Polynesia by Cabioch et al., 2008) are characterised by periods of reef accretion punctuated by major growth hiatuses. The causes of the shallow hiatuses have been correlated with meltwater pulses that would have induced reef drowning, including the terminal LGM ca. 19 ka (Lambeck et al., 2000; Yokoyama et al., 2001), MWP-1A ca. 13.8–14.7 ka (Fairbanks, 1989; Bard et al., 1996; Hanebuth et al., 2000; Webster et al., 2004; Fairbanks

484 Table 7 Spatial distribution of facies and biota by morphologic feature at each site. Facies are described in order of abundance and abbreviations are as follows: BD = boundstone, IC = isolated colony, RH = rhodolith, MC = macroid, IBD = indurated boundstone, CN = crystalline, GR = grainstone, FL = floatstone, RD = rudstones, SH = shellstone, CM = calcimudstone. Boundstone qualifiers include c = coral, f = foraminifera and a = algal. See Table 6 for facies descriptions. Modern corals and CCA are those that were collected with living tissue or confirmed modern through AMS and U–Th dating. Recent corals still retain their surface ornamentation. See text for classification as in situ. Site

Ribbon Reef

Viper Reef

Facies

Age range (ka)

Bioerosion/ Red Encrustation coralline (%) algae thickness

Corals Modern/recent

In situ fossil

Modern –2.0

Platy and encrusting Poritid (undet.), encrusting and platy Porites sp., encrusting and massive Favid (undet.), platy and tabular Galaxea paucisepta, encrusting and platy Agaricid (undet.), encrusting and platy Pachyseris speciosa, tabular and platy Acroporid (undet.), encrusting Montipora, sp. Massive and encrusting Porites Massive and encrusting Porites sp., sp., massive Favid (undet.), massive Goniopora sp.?, massive columnar Cyphastrea Favid (undet.), massive Cyphastrea chalcidium, encrusting plates chalcidium, encrusting and massive and robust branching Acropora Agaricid (undet.), encrusting plates of sp., encrusting Montipora sp., Acropora sp., encrusting Montipora sp. encrusting Agaricid (undet.)

Shelf break (70–82 m)

BDca, BDfc and BDfa with rare MC, IBD and FL

60/80

1–11 mm

Porites sp., Leptoseris sp., Fungia sp.?, Pachyseris sp.?, Montipora sp.

Outer barrier reef (46–51 m)

BD (fc,ca, Modern–? 55/90 fa) and IC with rare MC

1–10 mm

Porites sp., Acropora sp., Montipora sp.?, Pachyseris speciosa, Pavona sp.?, Seriatopora sp., Lobophytum sp., Favid (undet.)

Upper slope (98–120 m)

IBD and FL with rare BDc, IC, RD

10.1–11.8

30/15

1 mm

Shelf Break (100–102 m) Terrace rim (87–91 m) Outer barrier reef (53–61 m)

IBD and CN IBD

None None

BD (ca, Modern fa, c, f, fc), –7.8 IC and CN with rare MC, IBD, FL, RD and SH.

60/75

1–3 mm

Submarine landslide (159–173 m)

IBD

25/40

1 mm

16.3–14.0

Red coralline algae and associated sediments Reworked fossil

Massive, platy and encrusting Porites sp.; encrusting and platy Favid (undet.a), platy Galaxea paucisepta; massive Montipora sp.

Modern

Fossil

Mesophyllum sp. and thin melobesioidb

Lithoporella sp.?, Mesophyllum funafutiense, interlayered Mesophyllum sp. with Lithoporella sp., Peyssonnelia sp., thin laminar Lithothamnion sp.

Encrusting Porites sp., encrusting Pachyseris speciosa

None

65/30

None

None

Lithothamnion muelleri?, Lithothamnion prolifer, Mesophyllum sp., Peyssonnelia sp. and thin melobesioid, Hydrolithon breviclavium, Hydrolithon sp.?, Lithophyllum cuneatum, Lithophyllum acrocamptum, Lithophyllum insipidum, Lithophyllum gr. pustulatum, Lithoporella sp., Neogoniolithon fosliei, Neogoniolithon sp., Sporolithon molle, Peyssonnelia sp., vermetid gastropods N/A Hydrolithon rupestre?, Mesophyllum funafutiense, laminar Lithothamnion sp., laminar melobesioid, Peyssonnelia sp., Paulsilvella sp., Halimeda sp., pelagic infilling N/A N/A

65/40

None

None

N/A

Massive Porites sp., encrusting and massive Montipora sp., platy Pachyseris speciosa

Massive, platy and encrusting Hydrolithon sp.?, Mesophyllum sp., Porites sp., massive Goniopora sp., Mesophyllum funafutiense, thin branching Echinopora sp., tabular melobesioid Galaxea paucisepta, platy and massive Agaricid (undet.), platy Pachyseris speciosa, platy Montipora sp.

Laminar Lithothamnion sp., Hydrolithon reinboldii ??, Lithoporella sp., Halimeda sp.

None

None

Lithoporella sp., Peyssonnelia sp., Mesophyllum funafutiense, Spongites?, Lithophyllum sp.?, peloidal sediments, hemipelagic sediments

Pachyseris sp., Montipora sp., Echinopora sp., Pavona sp.?, Leptoseris sp., Galaxea astreata, Acropora sp., Pachyseris speciosa, Montipora sp. Octocoral (undet.)

N/A

N/A


Noggin Pass

Feature

IBD with 11.1–12.3 rare BD (ca, fc, c), IC and CM

50/50

1–3 mm

Shelf break (101–112 m)

IBD, CN and IC with rare BD (c, a, b), FL and SH BD (fa, f) and CN with rare MC, IBD and IC MC, IC, BD (fc, ca) and CN with rare RH

6.0–9.5

65/65

1 mm

Modern –5.5

75/85

1–2 mm

Modern–? 70/95

1–2 mm

10.2–12.0

35/85

1 mm

10.5–11.2

55/85

None available

45/65

10.2–13.1

55/65

1–2 mm

IC and BD 10.2–12.2 (ca, c) with rare CN, RH and MC

45/90

1–10 mm

Terrace (93–94 m)

Upper shelf (57, 66–69 m)

Hydgraphers Upper slope Passage (127–133 m)

Upper slope ridge (126–127 m) Shelf break (103–110 m) Terrace rim (86–92 m)

Outer barrier reef (52–53 m)

a b

IC, CN, BD (b, c, ca) and FL with rare GR, RD, SH, IBD and CM Rare IC, RD and CN Rare IBD, IC, FL and CN IC, FL, CN, BD (c, ca, b) and RD with rare IBD, CM and GR

Encrusting and massive Porites sp., encrusting plates of Acropora sp., encrusting Montipora sp., massive Favid (undet.), massive Cyphastrea sp. Encrusting Porites sp., platy Agaricid (undet.), platy Pachyseris speciosa

Favid (undet.), Porites sp., Agaricid (undet.), Montipora sp.

None

N/A

N/A

Platy Porites sp., encrusting and platy Agaricid (undet.)

Sporolithon sp. (no sori), Peyssonnelia sp., thin melobesioid

N/A

Encrusting Porites sp.

Encrusting Porites sp., platy Pachyseris speciosa

Lithothamnion muelleri?, Mesophyllum sp., Sporolithon sp.

N/A

None

N/A Peyssonnelia sp., Lithophyllum gr. pustulatum, Lithothamnion muelleri?, thin melobesioid, Halimeda sp.

None

Massive, platy, encrusting corals (undet.), encrusting, massive and platy Porites sp., massive Favid (undet.), massive and encrusting Agaricid (undet.), platy Pachyseris speciosa, platy and encrusting Montipora sp. Massive Poritid (undet.), encrusting and platy Porites sp., platy Montipora sp., massive Favid (undet.), tabular Galaxea astreata, platy Agaricid (undet.), platy Pachyseris speciosa Branching Echinopora sp.

Branching octocoral

Branching octocoral

Massive Montipora sp., massive Favid (undet.)

Branching, platy and massive corals N/A (undet.), massive, encrusting and platy Porites sp., massive Favid (undet.), tabular Galaxea paucisepta, massive Cyphastrea chalcidium, tabular Acroporid (undet.), encrusting plates and branching Acropora sp., platy and massive Montipora sp., platy Agaricid (undet.), branching Pavona maldivensis? Columnar Poritid (undet.), platy, Mesophyllum sp., Lithothamnion massive and encrusting Porites sp., muelleri?, thin melobesioid massive Favid (undet.), tabular Galaxea paucisepta, tabular Galaxea astreata, encrusting and massive Montipora sp., encrusting Agaricid (undet.), platy Pachyseris speciosa

Massive Porites sp., branching octocorals

Montipora sp., Acropora sp., Pachyseris speciosa Galaxea astreata, G. paucisepta., Seriatopora sp.

Platy Porites sp.

Thin melobesioid

Lithoporella sp., Mesophyllum sp.?, Lithophyllum pustulatum, oyster/ bivalve cement

N/A

N/A

N/A

N/A

Lithothamnion sp., Mesophyllum sp., laminar melobesioids, Hydrolithon reinboldii, Lithophyllum sp., Lithophyllum pustulatum?, Halimeda sp.


Upper slope (110–114 m)

Mesophyllum funafutiense

Undet = undetermined. Melobesioid = member of the Subfamily Melobesioideae, Family Hapalidiacea. 485

486


Autochthonous facies

Upper shelf

Calcimudstone Floatstone Rudstone Grainstone Crystalline Shellstone

Indurated boundstone Rhodolith

Macroid

25%

20%

65%

65%

Terrace rim

Terrace top Shelf break Upper slope

40%

65%

30%

50%

50%

70%

increasing depth

Isolated colony Boundstone Outer barrier reef 30% 40%

Allochthonous facies

25%

20%

80%

35%

50%

50%

Deep ridge +

Submarine landslide Fig. 8. Distribution of each facies by geomorphological feature. Selected concentrations of facies are indicated by text annotations for reference.

et al., 2005; Deschamps et al., 2012) and MWP-1B ca. 10–11 ka (Fairbanks, 1989, 1990; Bard et al., 1990, 1996). This mesophotic hiatus at 10–8 ka is not closely correlated with a known meltwater pulse (Fig. 13). Instead, the end of the first mesophotic generation about 10 ka is coincident with the initiation of modern reefs of the GBR shelf (Fig. 13) and the Gulf of Carpentaria (Harris et al., 2008), all of which developed on surfaces ca. 20–30 m. A recent study (Smith et al., 2010) shows that these marginal habitats can experience wide-scale mortality events while nearby shallow-water reefs show no evidence of disturbance, termed ‘cryptic mortality’. In the relatively depauperate Caribbean reefs, shallow reef drowning and backstepping have been recognised as indicators of a 6 m jump in sea-level. Small changes in sea-level have a more significant effect on the overall community structure of these reefs (Blanchon et al., 2002), which might explain why similar instances of reef drowning and backstepping over low-amplitude sea-level jumps have not been recorded in the highly diverse Pacific coral reefs over this same period (Blanchon, 2011). However, mesophotic coral communities of the GBR may be more susceptible to perturbations, as they have relatively lowdiversity and generally have only ca. 80–90 species represented (Bridge et al., 2011b) compared to 300+ living in the shallow environments. Mesophotic corals living at their maximum depth range may also be intrinsically more susceptible to mortality (Anthony and Connolly, 2004; Menza et al., 2007), and minor changes in water depth or quality can be devastating. A shelf-wide siliciclastic sediment flux has been well-discussed and constrained to 11–8 ka (Dunbar and Dickens, 2003; Page and Dickens, 2005). The authors correlated a dark and siliciclastic-rich horizon across a 2700 km north–south transect in sediment cores in the GBR, with the timing of sea-level crossing the shelf break and the remobilisation of sediments. During this same period, Webster et al. (2012) presented new palynologic evidence from the northern GBR confirming a strong mangrove signature within these horizons, indicating the presence of a mud source on some parts of the adjacent shelf. Mass accumulation rates of both siliciclastic and carbonate sediments peaked at about 10 ka across the shelf (Dunbar and Dickens, 2003) (Fig. 13) with elevated, but significantly lower rates found in the southern compared to the northern and central GBR (Page and Dickens, 2005). From 8 to 6 ka, sedimentation had again reduced.

In spite of the evidence for a wide-spread sediment flux, a clear indication of reduced water quality is not apparent from the fossil communities on the shelf edge. Leading up to the mesophotic coral hiatus, from 11 to 10 ka (and coincident with the start of the sediment flux at 11 ka), communities were characterised by relatively diverse corals, a high degree of encrustation (50–90%), and diverse and environmentally sensitive epibionts, including mastophoroid coralline algae, consistent with clear, oligotrophic waters of the GBR and other Indo-Pacific reefs (Gherardi and Bosence, 1999; Perry and Smithers, 2006). However, Celleporaria sp., which thrive in turbid and mesotrophic conditions (Hageman et al., 2003), was abundant locally at Hydrographers Passage. While the fossil biota alone do not show a clear record of increasing shelf edge sedimentation or turbidity, the close timing of the hiatus across the sites, and the observed period of maximum sediment flux off the shelf in the central and northern GBR provides compelling evidence of a causal relationship. Similar to modern coral reefs, where multiple factors act in concert to reduce reef resilience and induce demise (e.g., Anthony et al., 2011), the combined effects of regional perturbations of sea-level rise, sediment flux, increased turbidity and reduced light were most likely the factors responsible for the mesophotic reef demise and subsequent hiatus. Most of the mesophotic corals were growing at the maximum depth range of their modern counterparts immediately prior to the hiatus. At depths greater than 60 m, coral abundance is sharply reduced in the modern mesophotic community (Bridge et al., 2010; Bridge et al., 2011b), and the massive and tabular corals are absent based on observations. Declines in coral abundance coupled with diminished coral diversity, two effects of submergence beyond 60 m for these communities, are key factors in reducing the resilience of a reef (see Nystrom et al., 2008 for a review of ecological resilience). Additional extrinsic factors of sea-level rise coupled with the sediment flux would have compressed coral habitats, possibly reducing the depth of light attenuation by tens of metres. By 10 ka, wide swaths of the outer shelf may have become temporarily degraded during the transgression due to both the sediment flux and the increased depth. Reefs in palaeowater depths as shallow as 20–30 m on the outer shelf were affected (Webster et al., 2011), but those initiating in shallow-water conditions on the inner and mid shelf (Hopley et al., 2007) were apparently unaffected.


While the actual degree of turbidity during the sediment flux has not been quantified, mass accumulation rates are known to have risen by 4–5 times from 11 to 8 ka (Dunbar and Dickens, 2003; Page and Dickens, 2005). The rate of sea-level rise is also not wellconstrained during this period for the GBR, though the transgression most likely continued until at least 7 ka (Lewis et al., 2013). Finally, whether 60 m depth is the ‘tipping point’ for mesophotic coral communities of the GBR, or the sea-level rise and sediment flux perturbations were extreme events cannot be decoupled from this dataset alone. Based only on the estimated effects of each of these three factors (sedimentation, sea-level and community changes), the mesophotic reefs would have been subjected to extreme environmental stress that exceeded their limits of tolerance. Despite a chronologic data base of 67 high resolution ages, the lack of corals from 10 to 8 ka is not indisputable evidence of a hiatus. Great care was taken to avoid any sampling bias, but material that was poorly preserved with extensive bioerosion was avoided due to contamination risk. Platy and encrusting corals were generally more heavily bioeroded and massive corals less so, meaning the final and deepest drowning community (represented by the heavily bioeroded corals) may not have been radiometrically dated in each case. Further work to reduce the possibility of bias, including additional radiometric dating with a focus on the platy and encrusting coralgal assemblage would be worthwhile. Estimates of the paleo-turbidity as sediment was transported across the shelf (Dunbar and Dickens, 2003), along with more precise reconstructions of the rate and amplitude of sea-level rise during the mesophotic hiatus, would also provide important environmental constraints.

2.

3.

4.

4.4. Implications for modern mesophotic communities 5. Mesophotic communities are regarded as protected habitats, lessinfluenced by thermal stress and other disturbances (e.g., Bongaerts et al., 2010) than their shallow counterparts. However, studies show these deep communities are sensitive to not only some of the same perturbations as shallow-water reefs (e.g., Lesser and Slattery, 2011), but also to a unique suite of deep-water mortality events (e.g., Smith et al., 2010). The GBR fossil communities persisted through a lengthy period of environmental change and sea-level rise. During development from 13 to 10 ka, corals grew through deepening of about 30 m as well as the highest recorded volume of sediment flux across the shelf from 11 to 10 ka. To have tolerated such conditions might add weight to their interpretation as robust. However, ultimately these fossil mesophotic coral reefs died, when they became less tolerant and more vulnerable, as paleowater depth increased to about 60 m while at the same time being subjected to high rates of sedimentation. 5. Conclusions This study provides the first evidence that the submerged fossil corals reefs preserved along the shelf of the GBR also supported widespread fossil mesophotic coral communities at modern depths of 45–130 m. Based on their sedimentological, palaeo-ecological, and chronological characteristics, and compared with known sealevel and their modern distribution, these communities developed episodically in mesophotic environments from 13 ka. These are the first fossil mesophotic coral communities to be comprehensively studied in the GBR from the last deglaciation. They offer valuable insight into tolerance and thresholds of these marginal communities during changing environmental conditions and we conclude that: 1. Based on the recovered modern and fossil assemblages, two distinct coralgal assemblages and one non-coral encruster

6.

487

assemblage are present on the shelf edge. The first assemblage is characterised by massive and tabular Porites, Montipora and faviids with mastophoroids and lithophylloids and secondary or minor melobesioid CCA components. The second assemblage is characterised by much thinner (b 2 cm), platy and encrusting coral morphologies, especially Porites, Montipora and agariciids, with associated melobesioids and Sporolithon and minor lithophylloids and mastophoroids. The third assemblage is dominated by foraminiferal and algal crusts, including melobesioids, Peyssonnelia and Sporolithon only. Based on the modern distribution of the coralgal or non-coral encruster assemblage in each case, the massive/tabular community represents palaeo-water depths of b60 m, the platy/encrusting coral community represents b 80–100m, and the non-coral encruster community represents N100 m. Three distinct vertical patterns of overgrowth and biological succession are observed and represent stable photophilic (within the photic zone, b 100 m), photophilic-cryptic deepening (approaching the extreme edge of the photic zone at 100 m) and deeply submerged cryptic (N 100 m) environmental settings. Using 67 radiometric ages, two distinct generations of mesophotic coralgal community growth are identified, separated by a 2ka hiatus. The first generation occurred from 13 to 10 ka at depths of 100–130 m and exhibits clear deepening signatures through time. This resulted in the drowning of the massive coralgal assemblage at 11–10 ka in three of the four study sites as palaeo-water depths increased to greater than 60 m. A hiatus in coral growth followed between 10 and 8 ka as the non-coral encrusters assemblages dominated. The second coralgal generation occurred from 8 ka to present at depths of 95 m to at least 45 m but does not exhibit deepening signatures. Cessation of the first coral generation at 11–10 ka is coincident with modern reef initiation at ca. 30 m on the GBR as well as an increase in shelf-wide siliciclastic sediment flux. This suggests that coral communities are less resilient to perturbations when they are also persisting at their maximum depth tolerance, making them particularly vulnerable to other environmental changes. Conditions were sufficiently restored by 8 ka, resulting in the repopulation of the deep forereef slopes by similar mesophotic coralgal assemblages, with massive/tabular coral communities extending to 60 m, and platy/encrusting corals to 80–100 m.

Acknowledgements We thank the captain and crew of the RV Southern Surveyor for their outstanding work on the cruise. This research was funded by the Australian Marine National Facility, the National Geographic Society, Australian Research Council (DP1094001) and the Natural Environment Research Council (NE/F523318/1). Radiocarbon ages and XRD were funded by a postgraduate research grant awarded by the Australian Institute of Nuclear Science and Engineering and facilities were provided by the Australian Nuclear Science Technology Organisation. We acknowledge Paul Taylor of the Natural History Museum, London for his contribution to the bryozoan taxonomy. This paper is dedicated to the memory of Guy Cabioch, a friend and fellow reef worker. References Abbey, E., Webster, J.M., Beaman, R.J., 2011a. Geomorphology of submerged reefs on the shelf edge of the Great Barrier Reef: the influence of oscillating Pleistocene sealevels. Mar. Geol. 288, 61–78. Abbey, E., Webster, J.M., Braga, J.C., Sugihara, K., Wallace, C., Iryu, Y., Seard, C., 2011b. Variation in deglacial coralgal assemblages and their paleoenvironmental significance: IODP Expedition 310, “Tahiti Sea Level”. Glob. Planet. Chang. 76 (1–2), 1–15.

488 Table 8 Radiocarbon dating results. Biota are described in order of vertical succession (i.e. base to top, or inner to outer). T = transported or reworked, I.S. = in situ. Calibrated median ages are reported in years before 1950 CE. An age of 0 (zero) indicates that the radiocarbon activity is too high (young) to be calibrated using the MARINE09 calibration curve. Calcite quantification was only performed on corals. Pooled means are calculated for corals that underwent replicate radiocarbon dating (OZ lab codes only).

Ribbon Reef

Noggin Pass

Context — in situ (I.S.), transported (T)

Facies

Biota within specimen (bold* = dated)

D2 (46–50)

I.S.

BDca

Massive Favid*

D3 (70–82)

I.S. T

BDc BDfa

D4 (47–51)

T T T T

BDfa BDca BDca BDca

T

BDca

I.S.

BDca

Recently I.S.

BDca

I.S. T Recently I.S.

BDc BDca BDca

I.S.

BDca

I.S.

BDca

I.S. I.S. I.S. I.S.

BDfa BDfa BDca BDfa

I.S. T

BDb RD

T I.S. I.S.

FL BDc IBD

Crystalline texture then tabular Acropora* Interlayered CCA and Acervulinids then platy Agaricid, then interlayered red coralline algae (CCA)* and Acervulinids Interlayered acervulinids with algae* Platy Pachyseris speciosa then interlayered CCA* and Acervulinids Interlayered platy Pachyseris speciosa and CCA* Massive Acropora then thick CCA then interlayered thin Porites* and CCA Massive Favid then Porites then massive unknown coral then Acropora, then Acroporid Favid* then Porites then Acervulinid and thin Neogoniolithon?, Lithophyllum insipidum, Lithothamnion prolifer, Peyssonnelia and Hydrolithon? Peyssonnelia, Lithophyllum gr. pustulatum, vermetid grastropods then massive Porites* then Neogoniolithon and Lithophyllum insipidum interlayered then vermetid gastropods then thick Acropora with thick crusts of Hydrolithon? Encrusting Porites*(?) then coral(?) Porites then laminated Mesophyllum funafutiense* Bryozoan then encrusting Montipora then interlayered CCA* and Acervulinids Indurated sediments then 2 mm CCA then massive Porites then CCA then encrusting Porites then massive Montipora* then CCA then encrusting Montipora 3 mm CCA then massive Montipora then CCA* then bryozoan then CCA then massive Montipora then CCA then encrusting Porites Basal CCA* then thick Acervulinids Very thin CCA* then thick Acervulinids then indurated sediments Coral then thin CCA* Interlayered Acervulinids and CCA* then coral then Acervulinids interlayered with M. funafutiense, Peyssonnelia and laminar Lithothamnion Bryozoan* Parallel Halimeda* grains with Paulsilvellla fragments and thin laminar thalli Parallel Halimeda* grains Octocoral*, branching Laminated M. funafutiense, Lithophyllum, Spongites*

Dredge (depth m)

D7B (107–118) D8 (53–60) D9 (54–61)

D10B (107–118) D11A (98–104)

D11B (98–101)

Viper Reef

D14 (159–173)

14

C age

Calibrated age

370 ± 35 375 ± 40 490 ± 25 420 ± 30

30 40 90 0

935 ± 30 955 ± 25 2345 ± 35 0±0

530 540 1950 0

2σ range

Calcite (%)

Dating ID

90–0 0–100 1–150 0

0.3 0.2 0.8 N/A

OZL402u1 OZL402u3 OZL414 OZM203

480–610 500–610 1860–2070 0

N/A N/A N/A N/A

OZM201 OZL413 OZM202 CAM1

0±0

0

0

N/A

CAM2

33 ± 22

0

N/A

0

UBA-11,371

474 ± 22

70

1–150

0

UBA-11370

10,310 ± 60 155 ± 25 435 ± 25

11,290 0 0

11,170–11,440 0 0

b2 N/A N/A

OZN367 OZL411 OZL410

7185 ± 40

7640

7560–7730

b2

OZN368

7320 ± 40

7780

7670–7880

N/A

OZN369

10,005 ± 45 9995 ± 50 9405 ± 40 9250 ± 40

10,990 10,970 10,240 10,080

10,770–11,130 10,750–11,130 10,160–10,360 9910–10,180

N/A N/A N/A N/A

OZM200 OZM205 OZL405 OZM196

10,000 ± 45 10,550 ± 45

10,980 11,780

10,760–11,130 11,550–11,960

N/A N/A

OZM197 OZM198

10,260 ± 60 900 ± 35 12,550 ± 60 12,600 ± 60

11,230 510 13,980 14,040

11,130–11,380 440–570 13,810–14,160 13,810–14,240

N/A N/A N/A N/A

OZN370 OZM190 OZL407u1 OZM199

Paired U–Th ID

D4RR2ia, D4RR2ib


Site

D15 (101–112)

D16 (93–94)

D17 (66–69) D19B (110–114)

Hydro. Pass.

D21 (52–53) D22 (86–92)

D24A (127–133)

D24B (127–130)

D25A (126–127)

IBD IBD IBD BDa BDc MC BDfa BDa BDca

T I.S.

BDfc BDc

I.S.

BDc

I.S. I.S. I.S. I.S. T T I.S.

BDc IBD IBD IBD BDc FL BDca

I.S.

BDc

T

BDca

Recently I.S. T T T T T T I.S. T T

BDb BDb BDc BDc BDb BDc FL BDca BDb BDb

Laminated CCA* Thin basal CCA* Bryozoan* within hemipelagic matrix Laminated CCA* then bryozoan Indurated sediments then encrusting Porites* CCA* then thin Acervulinids then CCA Laminated interlayered CCA* and Acervulinids Laminated CCA* Thick laminated CCA* then encrusting Porites then thick laminated CCA then Acervulinids then Porites then interlayered CCA and acervulinids Agaricid then Acervulinids then CCA then massive favid* Massive Cyphastrea*

Indurated sediments then massive Porites then Montipora then laminated CCA* 3 mm Tabular Acropora*, 4 cm thick Basal CCA* then indurated sediments Basal CCA* then indurated sediments Basal CCA* then indurated sediments Massive Galaxea paucisepta* Halimeda* Hydrolithon reinboldii??, laminar melobesioids then massive Montipora* Massive Favid* then CCA and microbialite

Massive Cyphastrea chalsidium then Lithophyllum pustulatum?, Mesophyllum* Massive Cyphastrea chalsidium* then Lithophyllum pustulatum? Mesophyllum

Branching bryozoan* with Lithothamnion Bryozoan* Encrusting Porites*, Platy Galaxea astreata* Bryozoan* Massive Favid* Parallel Halimeda* grains Interlayered massive Porites* with CCA Bryozoan* Bryozoan*

13,700 ± 60 8505 ± 45 8625 ± 45 8950 ± 60 10,100 ± 70 10,590 ± 60 420 ± 25 440 ± 25 1380 ± 35 5190 ± 40

16,330 9120 9290 9610 11,100 11,840 0 0 920 5540

15,600–16,760 9000–9260 9130–9410 9470–9800 10,880–11,220 11,580-12,100 0 0 810–1010 5450–5630

N/A N/A N/A N/A N/A 1.5 N/A N/A N/A N/A

OZL407u2 OZN373 OZN372 OZN371 OZM181 OZM206 OZM193 OZM192 OZM191 OZN374

0±0 10,620 ± 60 10,640 ± 70 10,820 ± 70 10,080 ± 60

0 11,900 11,940 12,270 11,090

0 11,630–12,120 11,630–12,280 12,020–12,420 10,880–11,200

0.3 0.3 0.3 0.3 N/A

OZM180 OZL406u3 OZL406u2 OZL406u1 OZM184

10,770 ± 45 5620 ± 35 8510 ± 45 8870 ± 45 930 ± 35 6790 ± 40 9351 ± 35

12,190 6000 9130 9510 530 7310 10,190

11,990–12,360 5900–6130 9000–9270 9430–9630 480–610 7231–7406 10,110–10,250

0.5 N/A N/A N/A 0.3 N/A b2

OZM195 OZN375 OZN376 OZN377 OZM185 OZN378 UBA-11372

10,220 ± 60 10,403 ± 34 10,480 ± 70 10,520 ± 70 9340 ± 40

11,200 11,430 11,590 11,680 10,180

11,110–11,320 11,260–11,660 11,300–11,910 11,330–11,960 10,090–10,260

b2 b2 0.3 0.6 N/A

OZL404 u3 UBA-11373, OZL404 u1 OZL404 u2 OZL403 u4

D22HP15ia

10,530 ± 70 10,575 ± 35 10,680 ± 60 10,790 ± 70 11,664 ± 45 9335 ± 45 10,105 ± 40 10,670 ± 60 10,690 ± 60 10,350 ± 60 10,360 ± 50 11,490 ± 60 9675 ± 50 10,150 ± 50

11,710 11,830 12,020 12,220 13,160 10,180 11,130 12,000 12,040 11,340 11,340 12,970 10,530 11,160

11,350–11,990 11,640–12,020 11,740–12,270 11,950–12,400 13,070–13,300 10,040–10,270 11,030–11,200 11,730–12,260 11,830–12,310 11,200–11,650 11,210–11,640 12,740–13,120 10,410–10,630 11,070–11,230

0.3 b2 0.3 0.4 N/A N/A 1.4 1.2 N/A 0.3 N/A 0.8 N/A N/A

OZL403 u2 UBA-11373 OZL403 u1 OZL403 u3 UBA-11374 OZN379 OZM194 OZM186 OZM188 OZM187 OZN380 OZM189 OZN381 OZN363

D22HP4ia

D22HP2ia

D22HP13ia


D20B (104–109)

I.S. I.S. T I.S. I.S. T I.S. T I.S.

489

490


Table 9 U-series ages: aU–Th ages calculated assuming a closed system and no correction for initial 230Th, all ages are presented in thousands of calendar years before AD1950 (i.e. −0.011 ka = 1961 CE). b(234U/238U) at the time at which the coral grew was calculated using the uncorrected age. Corrected ages are based on a contaminant phase bearing Th with a (230Th/232Th) estimated from the geochemical database GEOROC (http://georoc.mpch-mainz.gwdg.de). Dating ID

D4RR2ia D4RR2ib⁎ D22HP2ia D22HP4ia D22HP13ia⁎⁎ D22HP15ia

Dredge (mbsl)

Age* (ka)

2σ

D4 (47–51) D4 (47–51) D22 (86–92) D22 (86–92) D22 (86–92) D22 (86–92)

0.3905 0.0785 10.50 12.08 19.29⁎⁎ 11.92

0.0017 0.0018 0.04 0.05 0.08 0.05

(234U/238U)initial**

2σ

230

Thinitial corrected ages*

230

(

1.1468 1.1455 1.1462 1.1467 1.1404 1.1451

0.0009 0.0009 0.0009 0.0009 0.0009 0.0009

Th/232Th)initial

0.6

1.0

1.5

0.276 0.026 10.44 12.08 19.24⁎⁎ 11.90

0.199 −0.011 10.40 12.08 19.17⁎ 11.89

0.103 −0.057 10.35 12.08 19.09⁎⁎ 11.87

⁎ Replicate of sample D4RR2ia. ⁎⁎ Specimen was a bryozoan, and when compared to AMS age of the same specimen (Table 8), open system behaviour is apparent and therefore the U–Th age is suspect. Adey, W.H., 1986. Coralline algae as indicators of sea-level. Sea-level research: a manual for the collection and evaluation of data. In: van de Plassche, O. (Ed.), Geo Books, Norwich, pp. 229–279. Anthony, K.R.N., Connolly, S.R., 2004. Environmental limits to growth: physiological niche boundaries of corals along turbidity–light gradients. Oecologia 141 (3), 373–384. Anthony, K.R.N., Maynard, J.A., Diaz-Pulido, G., Mumby, P.J., Marshall, P.A., Cao, L., HoeghGuldberg, O., 2011. Ocean acidification and warming will lower coral reef resilience. Glob. Chang. Biol. 17 (5), 1798–1808. Bak, R., Nieuwland, G., Meesters, E., 2005. Coral reef crisis in deep and shallow reefs: 30 years of constancy and change in reefs of Curacao and Bonaire. Coral Reefs 24 (3), 475–479. Bard, E., Hamelin, B., Fairbanks, R.G., Zindler, A., 1990. Calibration of the 14C timescale over the past 30,000 years using mass spectrometric U–Th ages from Barbados corals. Nature 345, 405–410. Bard, E., Hamelin, B., Arnold, M., Montaggioni, L., Cabioch, G., Faure, G., Rougerie, F., 1996. Deglacial sea-level record from Tahiti corals and the timing of global meltwater discharge. Nature 382 (6588), 241–244.

Coral

Red coralline algae

Beaman, R.J., Webster, J.M., Wüst, R.A.J., 2008. New evidence for drowned shelf edge reefs in the Great Barrier Reef, Australia. Mar. Geol. 247 (1–2), 17–34. Blanchon, P., 2011. Back-stepping. In: Hopley, D. (Ed.), Encyclopedia of Modern Coral Reefs. Springer, New York, pp. 77–84. Blanchon, P., Jones, B., Ford, D.C., 2002. Discovery of a submerged relic reef and shoreline off Grand Cayman: further evidence for an early Holocene jump in sea level. Sediment. Geol. 147 (3–4), 253–270. Bongaerts, P., Ridgway, T., Sampayo, E., Hoegh-Guldberg, O., 2010. Assessing the ‘deep reef refugia’ hypothesis: focus on Caribbean reefs. Coral Reefs 29 (2), 309–327. Bosence, D., 1983. Coralline algal reef frameworks. J. Geol. Soc. Lond. 140, 365–376. Bridge, T., Done, T., Beaman, R., Friedman, A., Williams, S., Pizarro, O., Webster, J., 2010. Topography, substratum and benthic macrofaunal relationships on a tropical mesophotic shelf margin, central Great Barrier Reef, Australia. Coral Reefs 30 (1), 143–153. Bridge, T.C.L., Done, T.J., Friedman, A., Beaman, R.J., Williams, S.B., Pizarro, O., Webster, J.M., 2011a. Variability in mesophotic coral reef communities along the Great Barrier Reef, Australia. Mar. Ecol. Prog. Ser. 428, 63–75.

Acervulinid foraminifera

Bryozoan

100 m

60 m

A) Stable photophilic

B) Photophilic-cryptic

C) Stable cryptic

Increasing water depth Fig. 9. Models of vertical biologic succession. A) Stable photophilic succession characterised by the alternation of corals and CCA. B) Photophilic-cryptic succession, characterised by photophilic biota, usually a coral with CCA, overgrown by cryptic biota, usually acervulinids or bryozoan. C) Stable cryptic succession characterised by alternating layers of acervulinids, CCA and/or bryozoan and the exclusion of photophilic corals.


491

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Location

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70 90

mbsl

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Ribbon Reef Noggin Pass Viper Reef Hydrographers Passage Barbados (Fairbanks 1989, Bard, 1990) Tahiti (Bard, 1996)

130 150 170 Dated biota coral

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algae bryozoan Halimeda

Closed=in situ Open=transported

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algal boundstone bryozoan boundstone coral boundstone coralgal boundstone foraminiferal-algal boundstone indurated boundstone rudstone floatstone

150 170 18000 16000 14000 12000 10000 8000

6000

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Years cal BP Fig. 10. Radiometric dating results symbolised by (A) the taxonomy and site of the dated material and (B) the facies (see Table 6 for facies descriptions and Table 3 for details of radiometric dating). Paired AMS–U–Th ages are indicated with an arrow and all others are AMS only.

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Bridge, T.C.L., Fabricius, K.E., Bongaerts, P., Wallace, C.C., Muir, P., Done, T.J., Webster, J.M., 2011b. Diversity of Scleractinia and Octocorallia in the mesophotic zone of the Great Barrier Reef, Australia. Coral Reefs 31, 179–789. Bridge, T., Scott, A., Steinberg, D., 2012. Abundance and diversity of anemonefishes and their host sea anemones at two mesophotic sites on the Great Barrier Reef, Australia. Coral Reefs 31 (4), 1057–1062. Brinkman, R., Wolanski, E., Deleersnijder, E., McAllister, F., Skirving, W., 2002. Oceanic inflow from the Coral Sea into the Great Barrier Reef. Estuar. Coast. Shelf Sci. 54 (4), 655–668. Burr, G.S., Galang, C., Taylor, F.W., Gallup, C., Edwards, R.L., Cutler, K., Quirk, B., 2004. Radiocarbon results from a 13-kyr BP coral from the Huon Peninsula, Papua New Guinea. Radiocarbon 46 (3), 1211–1224. Cabioch, G., Montaggioni, L.F., Faure, G., Ribaud-Laurenti, A., 1999. Reef coralgal assemblages as recorders of paleobathymetry and sea level changes in the IndoPacific province. Quat. Sci. Rev. 18 (14), 1681–1695.

Algae only Encrusting/platy coral Massive/tabular coral Bryozoan only Sea level (Lambeck and Chappell 2002)

0

Years cal BP Fig. 11. Age vs. depth of coralgal assemblages (if identified) or biota (if assemblage is unidentified). Coloured vertical bars represent palaeo-water depth range of the massive/tabular (red) and encrusting/platy (yellow) coralgal assemblages and dashed lines incorporate dredging depth error. Black vertical error is derived from dredging range and horizontal error is AMS 2σ. Arrows identify reworked limestones. Sea-level data by Lambeck and Chappell, 2001 is for indicative purposes only and may not be directly applicable to the GBR (see Yokoyama et al., 2006 for a discussion on deglacial sea-level calculations on the GBR). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Non-coral encruster assemblage

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Fig. 12. Schematic of the vertical succession of biota on a shelf edge profile across submerged reefs and terraces. Successions are based on radiometric dating. Drowned shallow-water reef generations (RG; sensu Montaggioni, 2005) form the foundation for later mesophotic communities. The first mesophotic generation was formed by massive corals at depths of 85–130 m. From 13.0 to 10.2 ka, massive corals were succeeded by encrusting and platy growth forms and ultimately by encrusting coralline algae and bryozoans. The second mesophotic generation was formed by encrusting and platy corals from 80 to 100 m and by massive corals from 45 to 60 m. This coral growth did not recommence until 7.8 ka, leaving a period of ca. 2 ky as a possible hiatus of mesophotic growth, coinciding with modern reef initiation on the GBR. Davies, P.J., Braga, J.C., Lund, M., Webster, J.M., 2004. Holocene deep water algal buildups on the eastern Australian shelf. Palaios 19 (6), 598–609. Deschamps, P., Durand, N., Bard, E., Hamelin, B., Camoin, G., Thomas, A.L., Yokoyama, Y., 2012. Ice-sheet collapse and sea-level rise at the Bolling warming 14,600[thinsp] years ago. Nature 483 (7391), 559–564. Druffel, E.R.M., Griffin, S., 1993. Large variations of surface ocean radiocarbon — evidence of circulation changes in the southwestern Pacific. J. Geophys. Res. C Oceans Atmos. 98 (C11), 20249–20259. Druffel, E.R.M., Griffin, S., 1999. Variability of surface ocean radiocarbon and stable isotopes in the southwestern Pacific. J. Geophys. Res. C Oceans Atmos. 104 (C10), 23607–23613. Dullo, W.C., Camoin, G.F., Blomeier, D., Colonna, M., Eisenhauer, A., Faure, G., Thomassin, B.A., 1998. Morphology and sediments of the fore-slopes of Mayotte, Comoro islands: direct observations from a submersible. Reefs and Carbonate Platforms in the Pacific and Indian Oceans.Blackwell Publishing Ltd. 217–236. Dunbar, G.B., Dickens, G.R., 2003. Massive siliciclastic discharge to slopes of the Great Barrier Reef Platform during sea-level transgression: constraints from sediment cores between 15°S and 16°S latitude and possible explanations. Sediment. Geol. 162 (1–2), 141–158.

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Shelf edge - IODP Exp. 325 corals Inner shelf Mid shelf Outer shelf

MWP 1A

Hopley et al. corals

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Mesophotic generation II

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Mesophoticcorals (this study)

40

Larcombe et al. corals

60 80 100

Highest siliciclastic flux

mbsl

19 ka

}

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160 180

20000

15000

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Years cal BP Fig. 13. Summary of age relationships between fossil mesophotic and shallow water reefs in the GBR. Points above the plot (Hopley et al. and IODP Exp. 325 data) lack depth data so are plotted for age only. The radiocarbon ages of corals presented in Larcombe et al. (1995) have been recalibrated using the same method as described in the text. Corals collected from interreef areas have been excluded. Hopley et al. (2007) ages have also been recalibrated here. Three IODP Exp. 325 ages were omitted due to clear evidence of post-mortem transportation. In a comparison with known meltwater pulses and environmental perturbations, the hiatus of the first generation of mesophotic growth (indicated) appears to be coincident in timing with the siliciclastic flux (yellow bar) (described in Dunbar and Dickens, 2003; Page and Dickens, 2005). Coral growth recommenced about 1 ky after the start of the flux on the modern reefs, and about 2 ky in the mesophotic communities.

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